It is desirable to utilise fast flowing water to generate electricity from submerged power generating turbines. In fast flowing water, these turbines require high integrity submerged turbine supports that will not be moved by the high current.
In most high current areas, a floor of a body of water, such as the seabed can be formed from a particularly hard rock formation rather than soft mud or sand. This is partly as a result of the fact that the fast current tends to scour soft mud and sand away from the seabed to reveal the base rock formation.
The combination of fast flowing water and a hard seabed precludes the use of jack-up type vessels. Jack-up vessels comprise a plurality of support legs on which a platform is mounted. The platform is vertically moveable up and down the support legs to account for changing water levels. This type of vessel generally uses a drill string to drill bores in the seabed. Piles can then be grouted into the drilled bores in order to attach a turbine support structure to the seabed.
However, a problem arises when the legs of a jack-up vessel initially contact a hard seabed because the legs tend to bounce on the hard rock floor and as a result can become damaged and even fracture. Consequently, it is extremely difficult to locate and secure a jack-up vessel in a region where there is a hard seabed formation such that they tend not to be used in such circumstances.
The use of a dynamically positioned (DP) vessel is also generally precluded in areas with particularly high current because it is difficult to ensure that the DP vessel remains on station in areas of high current. Furthermore, because of the amount of fuel necessary to stabilise a DP vessel at high current speeds, this option is particularly expensive and therefore undesirable.
Areas of high current speed also pose another problem for securing a subsea structure to the seabed. It is generally only practical to install a pile during the slack water time window when the tide is slowest. This time window can be of the order of less than one hour and it is therefore extremely difficult, if not impossible, to perform multiple drillings in such a time window.
A solution to the above problems is proposed in WO2008/125830. This document describes a surface vessel on which a structure to be attached to the seabed is located. An example of such a structure is a tripod support for an underwater power generating turbine. When the structure is on the surface vessel, individual drilling rigs are attached to each leg of the tripod which is to be piled to the seabed. A crane is then used to lower the structure, with drilling rigs attached to the seabed.
At the seabed, each drilling rig is then activated. Each drilling rig comprises a percussion drill which drills into the seabed and pulls down a pile behind the drill bit into the drilled socket. When the socket is drilled to its maximum depth, the drill bit is retracted leaving the pile in the seabed. The drilling rig is then detached and withdrawn to the surface. Grout is then pumped into the annulus between the tripod foot and the outside of the pile and also into the cylindrical hole defined by the centre of the pile to seal the pile into the seabed.
The method of WO2008/125830 suffers from several drawbacks:
1) The surface vessel must be particularly large to be able to support and lower a tripod structure to the seabed. Consequently, heavy lifting equipment such as a large crane is required on the vessel.
2) Once drilling is complete, the percussion drill must be retracted in order to pump grout into the pile and seal the pile in the seabed.
3) The only thing that holds the submerged structure to the pile is the grout disposed in the annulus between the structure foot and the outside of the pile. This joint could be prone to failure, particularly if high current washes grout away before it fully sets.
4) Repeated use of the percussion drill will result in wear and tear on the drill leading to increased maintenance and operation costs.
5) This system may require the use of an ROV. An ROV can generally only operate in currents of less than 1.5 knots which restricts the areas in which this system can be used.
6) If one of the drilling rigs fails, it is a complicated and costly operation to replace the rig on the seabed and conduct the piling operation.
GB2436320 proposes an alternative method. This document describes a method of lowering a structure to be attached to the seabed from a surface vessel to the seabed. The structure comprises several legs in which drill bits are disposed. The drill bits are pre-mounted in the legs on the surface and are then drilled into the seabed to attach the structure to the seabed. The drilling of the drill bits is accomplished by an arm which is lowered on to the structure and comprises a drill motor to drive the individual drill bits into the seabed. The arm is then rotated around the structure to drill each bit in sequence. An alternative embodiment describes mounting a structure having a plurality of arms and drill motors on to the structure to be attached to the seabed. Grout reservoirs are also provided on each leg of the host structure to enable the drill bits to be grouted into the seabed once they have been drilled.
The method and apparatus of GB2436320 suffers from the drawback that the surface vessel must be able to lift both the structure to be submerged and the drilling assembly together down to the seabed. This increases the size of surface vessel required and therefore the cost and complexity of a drilling operation. Furthermore, the only thing that holds the submerged structure to the drill bits is the grout disposed in the area between the feet of the structure and the outer surface of the drill bits. This joint could be prone to failure, particularly if high current washes the grout away before it fully sets. Also, the weight and complexity of the assembly is increased by providing grout reservoirs on the structure to be attached to the seabed.